Small bee-bee size silicon shot is often used in the manufacture of various semiconductor products, such as solar or technical grade silicon. The shot is produced in one application by initially melting a 3″ to 5″ chunks of silicon material via induction or other heating technology, and pouring the molten material into a tundish or crucible with a small hole or orifice in the bottom. The molten silicon flows through the orifice either by gravity or by applying a differential pressure to the orifice (positive pressure to the molten silicon surface or a vacuum under the bottom orifice). The molten silicon is cooled prior to collection or packaging, such as by a “water quench” or other technique. By controlling the size of the orifice and/or the differential pressure, the fluid flow through the orifice breaks up into molten beads according to governing laws and the Raylaigh phenomena with respect to the buoyancy, viscosity, and the thermal diffusivity within the fluid. However, silicon has a very high surface tension. As a result, the initial droplets are typically too large for the desired shot size, for example, being on the order of 5 to 10 mm while the desired shot size may be 0.1 mm to 4.0 mm in many applications. Accordingly, the beads must be broken up to form smaller particles, known as shot formation, prior to cooling and collection, where it is desirable to provide uniformity in the size distribution of the final cooled silicon shot particles.
The melting, shot formation, and cooling operations, moreover, ideally must prevent or inhibit contamination of the silicon material. In the case of solar grade or technical grade silicon for many applications, such as those using semiconductor crystal pullers, there are many fabrication specifications such as shot purity, where typical purity requirements range from parts per million to parts per billion. In particular, Boron and Phosphorous or other “p” or “n”-type dopant impurities are undesirable, as these impurities can contaminate or adversely dope the material so as to affect the current generating capability as in the photo voltaic industry, or such impurities may inhibit the proper formation of complete crystals for technical grade silicon used in the semiconductor industry. Ideally, the molten silicon should not come into contact with any materials other than primarily quartz or graphite for these applications.
Conventional shot formation and cooling techniques often result in formation of various residues on the surface of the shot. Water quenching, in this regard, is expensive in view of the impurity requirements as the cooling water has to be recycled to eliminate impurities. When cooling with water, moreover, the resulting shot has been found to be very porous and may include entrapped water vapor or other gasses. Attempting to remelt such material in subsequent applications often results in undesirable sputtering and spitting. Water quench, moreover, generally fails to provide controllable shot size and uniform size distribution. Conventional attempts at using high pressure gas for atomization typically yield shot product that is entirely too small for subsequent processes, generally in the micron range, and this technique has heretofore been subject to impurities. Moreover, prior gas cooling attempts fail to provide uniform size distribution, and instead typically yield a wide variation of droplet sizes in which the smaller particles may be characterized as a spray. In addition, the use of cooling air is undesirable because it will generally oxidize the silicon shot, thereby reducing its utility in technical or solar grade silicon applications.
Thus there remains a need for methods and systems for non-contact heat transfer to break the molten material beads into smaller particles, cool the material, and to transport it to a collection area in a controlled fashion to minimize exposure to impurities while providing uniform particle size distribution.